Gas-solid reaction processes for the removal of hydrogen halides from flue gases: A review

The management of hydrogen halides (HX) in industrial flue gases has gained renewed attention due to increasingly stringent emission regulations worldwide. Dry treatment processes that utilize the injection of solid sorbents are effective methods for separating HX from flue gases in various sectors. Their environmental and economic sustainability critically depends on their optimization, requiring an improved understanding of the fundamental physicochemical phenomena governing the gas-solid reaction process. Thus, a comprehensive review of experimental studies on the use of calcium-, sodium-, and magnesium-based sorbents for dry HX treatment is proposed, tracking the state of the art to identify knowledge gaps, guide future research, and support current industrial practice. Data from different sources are systematically examined to elucidate the influence of conditions such as temperature, humidity, and gas composition on sorbent performance. Distinct reactivity profiles among sorbents are highlighted, with Na-based reactants exhibiting superior HCl removal efficiency at low temperatures, Ca-based reactants benefiting from moisture and surface area enhancements, and dolomitic sorbents emerging as the most effective for high-temperature applications. Critical challenges in the modelling of gas-solid reactions for HX removal are analyzed, focusing on the limitations of conventional approaches, such as the inability to fully capture incomplete sorbent conversion in Ca-based reactants. The investigation of sorbent performance towards HF and HBr, key pollutants in emerging flue gas treatment markets, and the characterization of competitive and synergistic phenomena involved in the simultaneous removal of multiple halides are identified as the main research needs to advance dry sorption in industrial flue gas cleaning.